This invention relates to disposable absorbent articles such as disposable diapers, sanitary napkins and the like.
Disposable diapers provide substantial advantages in convenience over diapers intended to be laundered and reused, particularly when the diapers are used away from home. In recent years, many different disposable diapers have been proposed and some have been successful in the marketplace. However, even the successful diapers are not altogether satisfactory in such functions as keeping moisture away from an infant's skin and in absorbing relatively large amounts of urine. In spite of this, these diapers have been a commercial success because consumers have been willing to accept their inadequate performance as part of the price for convenience.
In one form of a prior disposable diaper, creped cellulose wadding is used as the absorbent material, covered with a permeable paper-like facing material on the side to be brought into contact with the infant's skin and covered with an impervious plastic sheet on the outside. In such a diaper, the wadding becomes more or less uniformly saturated with urine as the infant voids, and thus a substantial amount of moisture is only a paper's thickness away from the infant's skin. In use, the weight of the infant presses the paper-like facing layer against the saturated wadding so that substantial amounts of moisture are expressed from the diaper and pass through the facing and into contact with the infant's skin.
In attempting to increase the ability of the disposable absorbent diaper to keep moisture away from the portion of the diaper which comes into contact with an infant's skin, it has been proposed to loosely distribute water-insoluble but water-absorbent particulate material into the absorbent region of a disposable diaper. Such particulate matter is disclosed in U.S. Pat. No. 3,669,103 to Harper, et al., U.S. Pat. No. 3,670,731 to Harmon, and U.S. Pat. No. 3,783,872 to King. However, such particulate matter tends to swell and form a gelatinous layer which, while absorbent, tends to block the liquid passage therethrough. As a result, the full absorptive capacity of the article is not utilized because liquid to be absorbed cannot reach the absorbent material. In addition, the loosely distributed particulate matter tends to migrate to the bottom of the absorbent region of the diaper, further reducing its efficiency for its intended purpose.
An interesting method of anchoring water-insoluble but water-swellable particles in a fibrous material, such as in the absorbent layer of a disposable diaper, has been proposed in U.S. Pat. No. 3,901,236 to Assarssan, et al., which concerns a method for producing hydrogel particles which have a fibrous coating. The patentee maintains that the fibrous coating on the hydrogel particles will interlock with the fibers in a fibrous material in which the hydrogel particles are deposited, such that the hydrogel particles will be prevented from migrating during handling of the fibrous material.
U.S. Pat. No. 3,888,256 to Studinger, presents another method of utilizing the absorptive capacity of the water-absorbent particulate material. It provides for a diaper in which a particulate swelling substance is arranged exclusively or additionally in one of the layers lying near that surface of the diaper which is adjacent the baby's skin when the diaper is worn, i.e., the diaper facing sheet. The particles are distributed throughout this layer in an arrangement such that the spacing between particles is no greater than half of the particle diameter increase which occurs upon swelling of the particles. This arrangement allows the initial flow of urine to pass through the top particulate-containing layer into the absorbent layer. While this is occurring, the absorbent particles are absorbing liquid at such a rate that by the time the liquid has passed to the absorbent layer, the particles will have swelled to form a barrier layer, thus preventing the liquid from passing back through the diaper facing and in contact with the skin when the wet diaper is compressed. Ideally, this barrier layer should prevent the back flow of liquid even when substantial pressure is applied to the diaper, such as when an infant sits down on the diaper.
The top layer of U.S. Pat. No. 3,888,256 is formed by adhering the particles to a surface of a sheet-form liquid-pervious carrier in a two-step process which consists of treating the liquid-pervious support sheet with an adhesive, and then sprinkling the layer of adhesive with the particles. Inasmuch as a portion of the particle surface is necessarily in contact with the adhesive, the total surface available for liquid absorption is materially reduced.
U.S. Pat. No. 3,890,974 to Kozak discloses a diaper containing a slitted hydrogel film which functions in the same manner as the facing of the diaper shown in the aforementioned U.S. Pat. No. 3,888,256. The slitted hydrogel film is disclosed as being a hydrophilic, substantially water-insoluble film disposed between the top sheet of a diaper and the backing sheet. The film contains a plurality of short slits, i.e., 1/16 to 3/8 inch in length, so that aqueous liquid contacting the film causes the film to swell and to thereby open the slits to permit passage of liquid therethrough. This film initially allows the passage of liquid through the slits, and by carefully controlling the length of the slits, prevents the liquid from passing back through the slits in the opposite direction. It is disclosed in this patent that the length of the slits must be carefully controlled to accomplish this result. An obvious disadvantage of this method is that a high degree of quality control is essential to the accomplishment of the purpose set forth in the patent.
In Weaver, et al., "Highly Absorbent Starch-Based Polymer", presented to the International Nonwovens and Disposable Association in Washington, D.C., on March 5-6, 1974, it is reported that graft copolymers of starch and polyacrylonitrile may be hydrolyzed by a base to convert the nitrile functionality thereof to a mixture of carboxamide forms and alkali metal carboxylate forms. According to Weaver, et al., while the alkali metal carboxylate form of the graft copolymer had only limited solubility in water, dispersions of the carboxylate form thereof dry to continuous films on various substrates. These dried films do not redissolve on addition of water, but instead swell tremendously, imbibing several hundred times their weight in water. Weaver, et al., further reports that particles of this type of absorbent material have been incorporated into absorbent batt layers of diapers in powder form.